Susceptibility of Some Common Pecan Rootstocks to Infection by Xylella fastidiosa
Pecan [Carya illinoinensis (Wangenh.) K. Koch], a hardwood tree native to North America, is grown for commercial production of nuts in southeastern, central, and western regions of the United States. Pecan is also grown commercially in Mexico, South Africa, Australia, and some countries in South America. Pecan trees can be infected by the broad host range xylem-limited bacterial pathogen Xylella fastidiosa Wells et al. Infection incites a leaf scorch disease that can cause significant defoliation, reduced tree growth, and lower nut yield. Pecan cultivars are clonally propagated onto rootstocks grown from open-pollinated seed of selected cultivars. X. fastidiosa is transmitted at a high frequency from infected rootstocks into newly developing grafted trees. Rootstocks resistant to infection would be beneficial to pecan nurseries and pecan producers to prevent infection of young trees through grafting, especially when combined with hot-water treatment of scions to eliminate the pathogen. Some common rootstocks were tested for variation in susceptibility to infection using mechanical inoculation with the pathogen. No outstanding level of resistance to infection was detected among the seven rootstocks tested. The rootstocks from 'Curtis', 'Elliott', and 'Riverside' were less susceptible than one standard rootstock in the test ('VC1-68') and less susceptible than highly susceptible 'Cape Fear' rootstock. Conversely, the rootstocks from 'Apache', 'Moore', 'Stuart', and 'VC1-68' seed had a level of susceptibility to infection comparable to 'Cape Fear' and perhaps are not the best choice for rootstocks in geographic areas where X. fastidiosa is prevalent. The results of this research suggest that there is variation in rootstock susceptibility to infection by X fastidiosa. The use of mechanical inoculation may facilitate identification of susceptibility categories of pecan rootstocks to the pecan bacterial leaf scorch pathogen.